Photodiode-transimpedance amplifier combinations are used in a laser power control circuit for DVD applications. The first order limitation to achieving high speed in combination with high precision in this circuit lies with the magnitude of the photodiode parasitic capacitance. As shown in FIG. 1, traditional circuits use the feedback resistor and diode capacitance time constant to set the dominant pole of the circuit. This time constant, divided by (1+AVOL), where AVOL is the open loop gain of the amplifier, sets the bandwidth of the circuit. Note that the internal pole of the amplifier becomes the secondary pole.
From a speed and accuracy viewpoint, this circuit has several limitations. First, the open loop gain is limited by the need to have the secondary pole, contributed by the amplifier, out far enough in frequency so as to not contribute excessive phase shift in the closed-loop response, which leads to instability. On the other hand, limited open-loop gain limits the maximum value of the transimpedance-setting resistor, RF, in the feedback if we are to achieve a given bandwidth.
A possible solution to this problem is utilization of a current-mode-feedback amplifier, (CMFA), as the transimpedance amplifier. CFMA's have inherently low open loop input impedance (looking into the emitters of a complementary NPN, PNP pair). As a result, the secondary pole formed by the diode capacitance and the input resistance of the amplifier is an order of magnitude away from the required amplifier bandwidth and has no effect on circuit response. As a result, the amplifier can be compensated internally by conventional means and the magnitude of the open loop gain AVOL, is not limited.
However, the CFMA approach has significant accuracy limitations. CFMA's have extremely high input offset voltages and bias currents compared to voltage feedback amplifiers and this largely prevents their use in this application.